The invention relates to a membrane carburetor for an internal combustion engine having a start-assist device.
In a membrane carburetor for an internal combustion engine, a control chamber of the engine is delimited by a membrane and is connected to a fuel tank of a work apparatus via a feed line. The engine is preferably a drive unit for a portable handheld work apparatus such as a motor-driven chain saw and the like. The control chamber is filled with fuel during operation of the engine and is connected via at least one intake opening to an air-intake channel of the carburetor to the engine. An underpressure arises in the control chamber because of the removal of fuel. This deflects the membrane whereby the feed line can be cleared. Conventionally, a check valve is arranged in a feed of the intake openings. This check valve prevents a penetration of fuel or of the air/fuel mixture from the air-intake channel into the control chamber.
For a membrane carburetor of this type, European patent publication 0,786,591 suggests a start-assist device which includes a scavenging pump and an injection pump. The scavenging pump is in a return line between the control chamber and the fuel tank. The control chamber can be flooded with fuel in advance of starting the engine by actuating the scavenging pump. In the known arrangement, the scavenging pump is mounted in the feed line to the control chamber. The injection pump includes a displacement piston which can be pressed into the control chamber whereby fuel from the filled control chamber is pressed through the intake openings into the air-intake channel. The displacement piston is coupled to an additional membrane which delimits the control chamber and reduces the volume of the control chamber when the rod is pressed in. A quantity of fuel, which corresponds to the volume reduction, is injected into the air-intake channel. In this way, and before the engine is taken into service (that is, before the engine can develop a suction action), fuel is made available in the air-intake channel in order to ensure the first ignitions of the engine.
Furthermore, a membrane carburetor is known wherein the scavenging pump is mounted in the return line between the control chamber and the fuel tank (see Walbro WYK Carburetor, Service Manual 1994). An underpressure can be generated in the control chamber with the scavenging pump. The air is pumped out of the control chamber in preparation for the start and the control membrane opens the fuel feed line because of its deflection when there is an underpressure so that fuel flows into the control chamber. This flooding operation of the control chamber in preparation for the start of the engine is also known as purging. The scavenging pump includes an elastic purging bellows which can be pressed inwardly. The injection pump of the known WYK membrane carburetor includes a pin-like displacement piston which can be pressed into the fuel filled control chamber in order to inject fuel into the air-intake channel. The injection operation by pressing in the displacement piston takes place after flooding of the control chamber by purging and is also known as xe2x80x9cprimingxe2x80x9d.
A metering of the priming fuel quantity to be injected is necessary in order to start the operation of the engine without difficulty and to ensure a runup until the suction action of the engine automatically draws the fuel from the control chamber into the air-intake channel. In the known carburetors, and for starting the engine, substantial experience of the operator is required in order to meter the fuel quantity, which is to be injected with the injection pump. Most often, too little fuel is injected so that the runup of the engine is not ensured and the engine goes to standstill after only a few revolutions. Furthermore, often too much fuel is metered so that an overenriched mixture is present in the engine.
It is an object of the invention to improve a membrane carburetor of the kind described above so that a reliable start of the engine is ensured with only simple manipulative handling by the operator.
The membrane carburetor of the invention is for an internal combustion engine including an engine for a portable handheld work apparatus. The engine includes a fuel tank, a fuel feed line connected to the tank and a fuel return line also connected to the tank. The membrane carburetor includes: a carburetor housing defining an air-intake channel communicating with the engine and through which a stream of air flowing in an intake direction is drawn by suction when the engine is operating; the carburetor housing including a control chamber connected to the fuel-feed line and to the return line; a membrane arranged in the carburetor housing and mounted therein so as to delimit the control chamber; a valve assembly operatively connected to the membrane for clearing the fuel feed line in response to a deflection of the membrane in response to an underpressure in the control chamber; an intake passage for connecting the control chamber to the air-intake channel and for conducting fuel thereto; the intake channel including outlet means opening into the air-intake channel; a check valve mounted in the intake passage; a scavenging pump defining a first start-assist device and being connected in the fuel return line between the control chamber and the fuel tank; an injection pump defining a second start-assist device; the injection pump including: a cylinder and a displacer piston delimiting a pump chamber into which fuel can flow; an injection line connecting the pump chamber to the intake passage; the displacer piston being longitudinally movable in the cylinder through a piston stroke for pumping fuel in the pump chamber through the injection line, the outlet means and into the air-intake channel; the injection pump further including: a stop for limiting the piston stroke; and, an adjusting element having a shape changing in dependence upon temperature to determine the length of the piston stroke.
According to the invention, the displacement piston of the injection pump is guided longitudinally in a cylinder and delimits therein a pump chamber which is connected to the feed of the intake openings via an injection line. The piston stroke of the displacement piston is delimited by a stop and is determined with respect to its length by the temperature-dependent configuration of an adjusting element. Depending upon the particular temperature, the maximum piston stroke and therefore the pump chamber volume is defined. During priming, it is provided that the entire pump chamber volume is pressed out by the displacement piston and injected into the carburetor air-intake channel for preparing the engine for starting. When operating the injection pump, the displacement piston is to be displaced in each case up to the stop whereby the simplest operability is ensured for precise temperature-dependent metering of the priming fuel quantity.
Bimetal elements are preferably provided as temperature-dependent actuating elements for the stop. The bimetal elements assume predictably different expansions and alignments in correspondence to their temperature. In an advantageous configuration of the invention, the displacement piston is mounted on the free end of a rod which can be pressed into the cylinder of the injection pump. A cylinder head plate forms the stop and the bimetal element is mounted between the displacement piston and the cylinder head. The cylinder head plate covers the cylinder and the rod extends through this plate. The bimetal element can be expanded in the stroke direction. The bimetal element is advantageously formed by a plate spring packet of bimetal plate springs connected in series.
The injection line of the injection pump advantageously opens into the feed of the intake openings of the carburetor air-intake channel next to and downstream of the check valve. In this way, the movable valve member of the check valve is wetted with fuel in advance of starting the engine. Thus, an unwanted sticking of the valve member to its valve seat is countered and the proper opening of the check valve is enabled when there is suction pressure present in the air-intake channel. The above problem of the valve member sticking to its valve seat occurs especially after the engine has been at standstill a very long time.